<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html {
      font-size: 1.5px;
    }
    /* rem: 相对于根元素(html)的font-size(默认16px); 移动端适配，流行vw */
    .box {
      width: 100rem;
      height: 100rem;
      font-size: 20rem;
      background-color: orange;
    }
  </style>
</head>
<body>
  
  <div class="box">
    我是box
  </div>

</body>
</html>